The present invention relates to a device for rotating a raster or a scanning-area on a target face of an image pickup tube by an electrical means.
By convention, a TV camera using a magnetic focusing and magnetic deflecting type tube such as a Plumbicon (trade name) which is an image pickup tube utilizing a photoconductive effect by using PbO for the target, employs a mechanical means for making a fine adjustment of the inclination of the raster on the target face. A deflection yoke including vertical and horizontal deflecting/focusing coils disposed surrounding the pickup tube is mechanically rotated about the axis of the image pickup tube. In a three-tube type color camera, unless red, blue and green rasters are completely registered to each other, color edging occurs in a picture. To prevent color edging from occurring, it is necessary to perform a precise and fine adjustment of the raster inclination of at least two of the three camera tubes. Such an adjustment and adjustments of deflecting/focusing conditions are called a registration. If an electrical means, in place of the conventional mechanical means, is used, the fine adjustment of the registration may readily and automatically be performed.
As is known, in the prior art the raster may be rotated by changing the current of the focusing coil of the image pickup tube or by changing the voltage of the third grid. However, when the raster is rotated by such current or voltage change, off-focusing of electron beams takes place as the raster is progressively rotated. Therefore, when the inclination of the raster is adjusted to an optimum position, there is a strong possibility that the resolution of a picture picked up by the image pickup tube is deteriorated.
To solve the problem, there is provided an electrical rotation system to rotate the electron beam focused onto the target by applying a static magnetic field around the target. Using the electrical rotation system, provided the registration never results in the off-focusing of the electron beam.
In FIG. 1, there is shown a schematic representation in cross sectional form of an image pickup tube to perform the electrical rotation. As shown, a rotation coil 10 is coaxially located in front of and along the periphery of the target face 12 of a pickup tube 14 around which a deflection coil 16 and a focus coil 18 are fitted. The rotation coil 10 applies a static magnetic field to rotate the scanning area on to the target face 12.
FIG. 2 shows a schematic representation illustrating a case where the electrical rotation is applied to the three-tube type color television camera. In the figure, reference numeral 20 designates a lens system for focusing an optical image of an object (not shown) onto the target; 22 a decomposing optical system (a dichroic prism, for example) for decomposing the optical image into images of the three primary colors (red, green and blue); 14R, 14G and 14B image pickup tubes for producing red, green and blue image signals; 10R, 10G and 10B rotation coils.
In the color television camera using the dichroic prism 22, the target faces of the image pickup tubes 14R, 14G and 14B are disposed closely to each other. With this arrangement, when a DC current flows in the rotation coils 10R, 10G and 10B, a DC magnetic field developed from the rotation coil 10R influences a portion of the target face of the tube 14G near the coil 10R, and a DC magnetic field by the rotation coil 10B influences a portion of the target face of the tube 14G near the coil 10B. Similarly, a DC magnetic field from the rotation coil 10G influences a portion of the target face of the tube 14R near the coil 10G and a portion of the target face of the tube 14B near the coil 10G. Thus, the influences of the DC magnetic field from the rotation coils make the rotation angles different in the upper portion and the lower portion of the respective image pickup tubes at the time of the correction of the scanning area rotation by the rotation coils of the image pickup tubes. Additionally, the drive currents of the rotation coils 10R, 10G and 10B include a weak noise and the noise by various electromagnetic waves induced in the lead wires of the rotation coils. Those noises induced in the output currents of the targets have an adverse influence on a picture, frequently leading to the deterioration of a picture quality.